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Title: Wavelength scaling of electron collision times in filament-produced plasma in solids
We report an anomalous regime of laser-matter interactions, which is created by the wavelength dependence of electron collision time during filamentation in solids. Experiments are performed using femtosecond-time-resolved interferometry by varying the filament driver wavelength from 1.2 to 2.3 μm and using a 0.8-μm probe. Information on the phase and absorption via interferometry enables simultaneous measurements of plasma densities and electron collision times during filamentation. Although it is expected that the plasma density decreases with increasing wavelength due to larger plasma-defocusing at longer wavelengths [1-4], our measured plasma densities are nearly constant for all the pump wavelengths. This observation is successfully explained by the measured wavelength-dependence of electron collision time: electron collision times in filament-produced plasma decrease with increasing wavelength, which creates an anomalous regime of plasma-defocusing where longer wavelengths experience smaller plasma defocusing. In addition, simulations with the measured electron collision times successfully reproduce the observed plasma density scaling with wavelength [5]. [1] L. Bergé et al., Phys. Rev. A 88, 023816 (2013). [2] Y. E. Geints et al., Appl. Opt. 56, 1397 (2017). [3] S. Tochitsky et al., Nat. Photonics 13, 41 (2019). [4] R. I. Grynko et al., Phys. Rev. A 98, 023844 (2018). [5] Nagar et more » al., submitted. « less
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Bulletin of the American Physical Society
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National Science Foundation
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  1. Abstract

    Although the dielectric constant of plasma depends on electron collision time as well as wavelength and plasma density, experimental studies on the electron collision time and its effects on laser-matter interactions are lacking. Here, we report an anomalous regime of laser-matter interactions generated by wavelength dependence (1.2–2.3 µm) of the electron collision time in plasma for laser filamentation in solids. Our experiments using time-resolved interferometry reveal that electron collision times are small (<1 femtosecond) and decrease as the driver wavelength increases, which creates a previously-unobserved regime of light defocusing in plasma: longer wavelengths have less plasma defocusing. This anomalous plasma defocusing is counterbalanced by light diffraction which is greater at longer wavelengths, resulting in almost constant plasma densities with wavelength. Our wavelength-scaled study suggests that both the plasma density and electron collision time should be systematically investigated for a better understanding of strong field laser-matter interactions in solids.

  2. We experimentally and theoretically investigate plasma dynamics in laser filamentation in fused silica by varying the driver wavelength from 1.2 to 2.3 μm covering the near-zero to the anomalous group-velocity dispersion regimes. First, we perform femtosecond time-resolved interferometry to measure plasma densities in filaments, which unexpectedly reveals that plasma densities are not monotonically decreasing with increasing wavelength. This result is in sharp contrast to recent theoretical work in filamentation in air/gases as well as our own numerical simulations in fused silica in which the electron collision time is assumed to be constant for all the wavelengths. Therefore, to investigate further, we also perform time-resolved shadowgraphy which, combined with interferometry, enables us to determine the electron collision time in plasma. We find out that the electron collision time is not a constant for different wavelengths, which can change the plasma dynamics in filamentation significantly.
  3. The multimodal carrier-resolved unidirectional pulse propagation equation is solved to study the wavelength-dependent (λ = 1, 2, 3 and 4 μm) spatio-temporal dynamics, particularly pulse self-compression during high-intensity laser pulse propagation in gas-filled capillaries. We find that pulse self-compression in gas-filled capillaries due to plasma is more efficient for short wavelengths in contrast to wavelength-dependent pulse self-compression in laser filamentation [1]. To explain our finding, a detailed analysis is performed by quantifying the contributions of higher-order modes and calculating the temporal delay among modes, which reveals that pulse self-compression at longer wavelengths does not occur due to larger group velocity mismatch between the fundamental and higher-order modes for longer wavelengths [2]. Our study has important implications for the various fields of high-intensity nonlinear optics in gas-filled capillaries such as supercontinuum generation and high-order harmonic generation [3]. [1] L. Bergé et al., Phys. Rev. A 88, 023816 (2013). [2] G. Nagar and B. Shim, submitted. [3] T. Popmitchev et al. Science 336, 1287 (2012).
  4. We present time-resolved interferometry to simultaneously measure plasma densities and electron collision times for strong field laser-matter interactions. First, an intense femtosecond pump pulse generates plasma in a solid and second, a weak 800-nm femtosecond probe traverses the pump-induced plasma and is sent to an interferometer with controlled time delay between pump and probe. By analyzing the interferograms using Fourie methods, we can extract plasma densities and electron collision times in plasma simultaneously with micrometer spatial and femtosecond temporal resolutions. Using the technique, we study the plasma dynamics when a wavelength-varied (λ= 1.2-2.3 μm) pump pulse undergoes laser filamentation in solid materials.
  5. Particle charging in the afterglows of non-thermal plasmas typically take place in a non-neutral space charge environment. We model the same by incorporating particle-ion collision rate constant models, developed in prior work by analyzing particle-ion trajectories calculated using Langevin Dynamics simulations, into species transport equations for ions, electrons and charged particles in the afterglow. A scaling analysis of particle charging and additional Langevin Dynamics calculations of the particle-ion collision rate constant are presented to extend the range of applicability to ion electrostatic to thermal energy ratios of 300 and diffusive Knudsen number (that scales inversely with gas pressure) up to 2000. The developed collision rate constant models are first validated by comparing predictions of particle charge against measured values in a stationary, non-thermal DC plasma from past PK-4 campaigns published in Phys. Rev. Lett. 93(8): 085001 and Phys. Rev. E 72(1): 016406). The comparisons reveal excellent agreement within ±35% for particles of radius 0.6,1.0,1.3 μm in the gas pressure range of ~20-150 Pa. The experiments to probe particle charge distributions by Sharma et al. (J. Physics D: Appl. Phys. 53(24): 245204) are modeled using the validated particle-ion collision rate constant models and the calculated charge fractions are compared with measurements.more »The comparisons reveal that the ion/electron concentration and gas temperature in the afterglow critically influence the particle charge and the predictions are generally in qualitative agreement with the measurements. Along with critical assessment of the modeling assumptions, several recommendations are presented for future experimental design to probe charging in afterglows.« less